The objective of this study is to understand the role of the uterine proteins in the reproductive physiology of mammals. Particular attention will be given to the synthesis of blastokinin in the rabbit, the steroids that regulate it, and the relationship between blastokinin and other uterine proteins especially esterases, reverse transcriptase and gonadotropin. Attempts will also be made to clarify the question of similar proteins in the human and luteotrophic factor in sheep.